The invention relates generally to a circuit for converting a signal from a capacitive sensor into a form suitable for digital signal processing. More specifically, the invention is a circuit for compensating for offset and sensitivity variations in the sensor output after the sensor has been calibrated.
Capacitive sensors are used in applications such as accelerometers and pressure sensors. One type of capacitive sensor is manufactured by micromachining a slab of silicon to form a miniature tilt plate suspended above a substrate by torsion arms. The tilt plate has a center of mass offset from the torsion arm axis such that, under conditions of non-zero acceleration perpendicular to the plate, the plate tilts relative to the underlying substrate. The tilt plate is metallized and forms a common electrode for two capacitors. Two metallized regions are formed on the substrate directly beneath the suspended tilt plate and form the other electrodes of two capacitors with the tilt plate electrode. The geometry of the two metallized regions and the tilt plate is such that under acceleration the capacitance of one capacitor increases while the capacitance of the other decreases since the distance between the tilt plate and the metallized regions on the substrate either decreases or increases.
In operation, a capacitive tilt plate sensor connects one capacitor electrode from each capacitor at a common node. The common node provides an output of the sensor and the two remaining electrodes provide inputs to the sensor. If one capacitance value is denoted by C.sub.A and the other capacitance is value C.sub.B, then the output of the sensor is given by the formula: ##EQU1## While the capacitances of C.sub.A and C.sub.B vary somewhat nonlinearly with respect to a acceleration, the above formula is remarkably linear with respect to acceleration.
Bias voltages are supplied to the input terminals of the sensor described above so that the output voltage of the sensor provides an indication of the magnitude of the acceleration. One problem with the above described sensor is that unequal bias voltages cause an electrostatic attraction which can result in a deflection of the tilt plate toward the metallized region below the plate. Electrostatic deflection reduces the accuracy of the sensor by changing the capacitance across the plate and inducing nonlinearities in the output.
It would therefore be desirable to reduce the effect of the electrostatic attraction of the tilt plate, thereby increasing the accuracy of the sensor.